Conversion of isobutane to isobutene

ABSTRACT

ISOBUTANE IS DEHYDROGENATED TO PRODUCE ISOBUTENE BY ESTABLISHING IN A FIRST O ZONE A DEHYDROGENATING AGENT CONSISTING ESSENTIALLY OF SULFUR AT 700-1000*C. AND THEN DIRECTLY CONTACTING SAME NON-CATALYTICALLY IN A SECOND ZONE AT 500-630*C. WITH ISOBUTANE. ESTABLISHMENT OF THE DEHYDROGENATING AGENT CAN BE DONE EITHER BY CATALYTICALLY REACTING SO2 AND H2S AT 700-1000*C. OR BY HEATING SULFUR TO 700-1000*C.

Jan. 22, 1974 BEAN 3,787,516

CONVERSION OF [SOBUIANE T0 ISOBU'I'ENF;

Filed Sept, 28, 197

m ZONE 700- mom 0 M203 CATALYST Q l l ISOBUTANEA1 SOD-630 c NON-CATALYTIC J TO ISOBUTENE SEPARATION United States Patent 3,787,516 CONVERSION OF ISOBUTANE T0 ISOBUTENE Roger M. Bean, Glen Mills, Pa., assignor to Sun Research and Development Co., Philadelphia, Pa. Filed Sept. 28, 1972, Ser. No. 293,193 Int. Cl. C07c 5/20 US. Cl. 260-6833 7 Claims ABSTRACT OF THE DISCLOSURE Isobutane is dehydrogenated to produce isobutene by establishing in a first zone a dehydrogenating agent consisting essentially of sulfur at 7001000 C. and then directly contacting same non-catalytically in a second zone at 500-630 C. with isobutane. Establishment of the dehydrogenating agent can be done either by catalytically reacting S0 and H 8 at 7001000 C. or by heating sulfur to 7001000 C.

BACKGROUND OF THE INVENTION This invention relates to the conversion of isobutane to isobutene by reaction at elevated temperature with a dehydrogenating agent.

Numerous references in the prior art disclose the use of sulfur-containing dehydrogenating agents for converting hydrocarbons to olefinic products at elevated temperatures. For example, an article by C. R. Adams, I. of Cat., 11, 96-112 (1968) and references cited therein show the dehydrogenation of isobutane and other hydrocarbons by reaction with sulfur dioxide at high temperatures, such as SOD-600 C., in the presence of various catalysts. The use of a combination of oxygen and various sulfur-containing compounds such as H 8 and S0 or a combination of H 8 with S0 or S0 for efit'ecting dehydrogenation reactions in the presence of catalysts is described in the following United States patents:

2,971,035, R. F. Stringer et al., Feb. 7, 1961 3,403,192, M. Vadekar et al., Sept. 24, 1968 3,585,249, A. D. Cohen et al., June 15, 1971 The use of sulfur as the agent for dehydrogenating hydrocarbons at elevated temperatures is disclosed in the following United States patents:

3,110,741, S. H. Patinkin et al., Nov. 12, 1963 3,247,278, W. E. Garwood et al., Apr. 19, 1966 3,585,250, I. S. Pasternak et al., June 15, 1971 Of this group only the Patinkin et al. patent contemplates carrying out the dehydrogenation reaction in the absence of a catalyst.

An article by H. E. Rassmussen et al., Ind. Eng. Chem., 38, 376,382 (1946) describes the non-catalytic reaction of sulfur with various hydrocarbons including isobutane. The procedure involved preheating separate streams of the hydrocarbbon and sulfur to a temperature of the order of 600 C., then mixing the streams and passing the mixture through a reaction zone maintained at an elevated temperature typically in the neighborhood of 650 C. and in the absence of any catalyst. Results reported in this article for dehydrogenating isobutane under these conditions show a conversion of about 23% with selectivity for isobutene production of about 65%.

SUMMARY OF THE INVENTION The present invention constitutes an improvement over the procedure of the last-mentioned reference, whereby "ice sulfur is utilized in a novel manner under non-catalytic conditions to secure a better conversion-selectivity relationship in the dehydrogenation of isobutane to isobutene.

It has now been found that if sulfur is first heated to or for-med at a temperature in the range of 7001000 C. in a first zone and then directly contacted in the absence of a catalyst with isobutane in a second zone maintained at 500-630 C., a distinct improvement in the conversionselectivity relationship is achieved as compared to that obtained with the non-catalytic procedure of the prior art. The process of the invention thus comprises establishing in a first zone the dehydrogenating agent consisting essentially of sulfur at a temperature in the range of 7 00-1000 0., preferably 750900 C., then directly admixing a stream of the sulfur with a stream of isobutane and passing the mixture through a second zone maintained at 500'- 630 0., preferably 525-575 C., and in the absence of catalyst to eifect dehydrogenation, cooling the reaction mixture and recovering isobutene therefrom as product. Establishment of the dehydrogenating agent in the first zone can be effected either by catalytically reacting S0 and H 8 at 7001000 C. or by heating sulfur to 700- 1000 C.

BRIEF DESCRIPTION OF THE DRAWING The accompanying drawing is a schematic illustration of the process of the invention in an embodiment wherein the dehydrogenating agent is formed by reacting hydrogen sulfide and sulfur dioxide at high temperature.

DESCRIPTION With reference now to the drawing, a first zone 10 is provided wherein sulfur is preparedby catalyzed reaction of hydrogen sulfide and sulfur dioxide at high temperature in accordance with the following equation:

The hydrogen sulfide and sulfur dioxide can be fed as separate streams to zone 10 as indicated in the drawing and therein heated to the desired temperature level of 7001000 0, preferably 750-900 C., and contacted with the catalyst. Alternatively the H 8 and .80 can be admixed and the mixture can be pre-heated to any temperature up to the desired temperature and then contacted at 7001000 C. with the catalyst in zone 10. The reaction of H 8 and S0 to form sulfur at somewhat lower tem peratures has been widely practiced in the Claus process utilizing alumina or bauxite as catalyst [see B. W. Gamson et al., Chem. Eng. Prog., 49(4): 203-215 (1953)], and the same kinds of catalyst can be utilized for the higher temperature reaction of the persent process.

An efiluent stream comprising the desired dehydrogenating agents passes directly from zone 10 through line 11 to the second zone 12 wherein it is admixed with a stream of isobutane from line 13. The isobutane should be preheated sufficiently so that the resulting mixture of isobutane and dehydrogenating agent will be at a temperature in the range of SOD-630 C., preferably 525- 575 C. The dehydrogenating agent just prior to mixing with the isobutane can be at the same temperature as zone 10 or the effluent stream from line 11 can be cooled to a level within the range of 500630 C. before admixing is effected. In the latter case it is desirable that the admixing occur immediately, e.g. within 10 seconds of the time the temperature of the effluent stream has dropped below 700 C., as a considerable time lapse before admixing the cooler efiluent stream with the isobutane may result in I reduced activity of the dehydrogenating agent.

The proportion of dehydrogenating agent to hydrocarbon should be such as to provide 0.1-5.0, more preferably 0.5-2.0, atoms of sulfur per molecule of isobutane. Low ratios of sulfur to isobutane tend to give high selectivities but low conversions. As this ratio is increased, the percent conversion tends to increase but selectivity concurrently drops.

Reaction in zone 12 between the isobutane and sulfur is etfected in the absence of a catalyst merely by maintaining the temperature in the range of 5 00-630 C., preferably 525-575 C., and allowing sufiicient residence time. The residence time in zone 12 can vary widely, with a generally useful range being 0.1-50 seconds and a more preferable range being 1-25 seconds. The pressure in the reaction zone conveniently can be about atmospheric although higher or lower pressures can be employed if desired. A small proportion of tar normally is formed in the reaction.

Under the foregoing conditions good conversion-selectivity relationships are obtained in dehydrogenating the isobutane to isobutene. Typical values are, for example, a selectivity of 90% at a conversion level of 25% or a selectivity of 67% at a conversion level of 45%. These values represent substantially more efiicient operation than shown in the prior art for the non-catalytic conversion of isobutane to isobutene.

The reaction products pass from the second zone 12 through line 14 through cooling means to a fractional distillation system (not shown) to recover isobutene from the product. Unconverted isobutane can be separated from any lower and higher boiling materials formed during the dehydrogenation reaction and recycled for further conversion.

In an alternative embodiment of the process elemental sulfur is used in place of H 8 and S0 The dehydrogenating agent in this variation is established merely by heating the sulfur in the first zone to a temperature in the range of 700-1000 C., preferably 750-900 C. If desired a mixture of steam and sulfur can be used, although this is not essential. The resulting dehydrogenation agent is then directly used in the second zone to effect conversion of the isobutane.

In each of the foregoing embodiments the dehydrogenating agent is established by first securing sulfur at a temperature level in the range of 700-1000 C. and then directly utilizing same for contacting isobutane non-catalytically at a temperature of SOD-630 C. As previously pointed out, this results in a better conversion-selectivity relationship than is experienced when the sulfur is heated to or formed at approximately the same temperature level at which the isobutane conversion is efiected. The reason for this improvement is not known with certainty but it is believed that it comes about because the higher temperature results in a form of sulfur having better activity, of which advantage is then taken by direct or immediate use of the active form at a lower tempera ture level that is optimum for the dehydrogenation reaction.

As specific illustrations of the invention, five runs were made in which sulfur was formed from H 8 and S0 at about 850 C. and then directly reacted with isobutane at about 550 C. Streams of H 8 and S0 in 2:1 molar proportion were admixed, and the mixtures were preheated and then contacted in a reactor tube with an eta alumina catalyst bed at about 850 C. A stream of the resulting dehydrogenating agent was directly passed to the top of another tubular zone containing no catalyst and maintained at about 550 C. and there admixed in selected proportions with a stream of isobutane. A stream of reaction product was continuously removed from the bottom of the tubular reactor. Table I presents data for each run after an on-stream period of minutes.

I.DEHYDRO GENATION OF ISOBUTANE VIA. SULFUR PREPARED FROM HzS AND S02 [Sulfur preparation temperature=850 C. Dehydrogenation temperature=550 0.]

TAB LE Resi- Isobutane Isobutene Szisodeuce convers ecbutane time, sion, tivity, Percent ratio 1 seconds percent; percent tar 1 Atoms 0i sulfiJr/molecule of isobutane.

The data in Table I illustrate the effect of varying the sulfurzisobutane ratio. At the lower ratios high selectivity for isobutene production can be obtained with relatively low conversion. Higher conversion can be attained at increased ratios but the selectivity decreases. However the conversion-selectivity relationship is substantially better than that achieved by the prior art non-catalytic process, wherein a selectivity of only 65% was obtained at a conversion level of 23%.

Table II presents data for two comparative continuous runs made in the same apparatus and in the same general manner as Runs 1-5 using sulfur made from H 5 and S0 Run 6 was made by reacting the H 8 and S0 at about 600 C. and utilizing the resulting dehydrogenating agent also at about 600 C. In Run 7 the H SSO reaction was carried out at 850 C. and the resulting sulfur was then utilized for isobutane dehydrogenation at 600 C.

TABLE II Isobutane dehydrogenations via sulfur formed at difierent temperature levels from H S and S02 1 Atoms 0t sulfur/molecule of isobutane.

As can be seen in Table II both runs gave the same selectivity under the respective conditions employed. However Run 7 in which the sulfur was prepared at the higher temperature level gave substantially higher conversion than Run 6, even though the residence time in the latter was considerably longer. These comparative data illustrate the improved results attainable with the present invention.

When runs are made essentially duplicating the foregoing except that the dehydrogenating agent is established by heating sulfur to the same temperature as used for the H S-SO reaction, substantially equivalent results are obtained.

The invention claimed is:

1. Method of converting isobutane to isobutene which comprises:

(a) establishing in a first zone a dehydrogenating agent consisting essentially of sulfur at a temperature in the range of 700-1000 C.;

(b) directly admixing a stream of said dehydrogenating agent from the first zone with a stream of isobutane and passing the mixture in the absence of a catalyst through a second zone maintained at 500- 630 (3., whereby dehydrogenation of isobutane occurs;

(c) cooling the reaction mixture from the second zone and recovering isobutene therefrom as product.

2. Method according to claim 1 wherein step (a) is effected by catalytically reacting S0 and H 3 in said first zone at 700-1000 C.

' 3. Method according to claim 2 wherein the H 8 and S0 are reacted at 750900 C.

4. Method according to claim 3 wherein the temperature in said second zone is in the range of 525-575 C.

5. Method according to claim 1 wherein step (a) is effected by heating sulfur to 700-1000 C.

6. Method according to claim 5 wherein the sulfur is heated to 750900 C.

7. Method according to claim 6 wherein the temperature in said second zone is in the range of 525-575 C.

References Cited UNITED STATES PATENTS 3,586,732 6/1971 Guth et a1. 260-6833 5 PAUL M. COUGHLAN, JR., Primary Examiner V. OKEEFE, Assistant Examiner 

